Microsensor with a resonator structure

ABSTRACT

A microsensor with a resonator structure, which is excited by first electrical signals to oscillate and emits second electrical signals in dependence on the measuring variable, wherein a heating element, supplied with at least one of the first electrical signals, is arranged on the resonator structure for the thermal excitations of oscillations. For the thermal excitation of lateral oscillations in a microsensor with a resonator structure, the microsensor is provided at one oscillating part of the resonator structure with at least two regions that are thermally separated by a zone with reduced heat conductance, and the heating element is arranged on one of the regions. This type of arrangements permits the excitation of the resonator structure to lateral oscillations if the heating element is supplied with corresponding current pulses. It is advantageous if a receiving element is arranged on at least one of the other regions to detect the oscillation amplitude.

BACKGROUND OF THE INVENTION

The invention relates to a microsensor with a resonator structure, whichis excited by first electrical signals to oscillate and emits secondelectrical signals in dependence on the measuring variable, wherein forthe thermal excitation of oscillations a heating element is arranged onthe resonator structure to which is supplied at least one of the firstelectrical signals.

An acceleration sensor with a resonator structure that is thermallyexcited to oscillate is known from “An Integrated Resonant AccelerometerMicrosystem for Automotive Applications,” Transducer '97, pages 843-846.With this known acceleration sensor, a seismic mass is suspended on twoarms in the center of a recess in the silicon chip. The two arms extenddiagonal. A resistor is arranged on one of the two arms, which issupplied with a pulsed current. Owing to the thermal stress that changesover time as a result of the temperature differences between the top andthe bottom, the structure is excited to oscillations that are adjustedto a resonance requirement by means of an outside wiring. A forceexerted onto the seismic mass, for example resulting from an externalacceleration perpendicular to the surface of the structure, causes achange in the resonance frequency, which is detected and evaluated.

The thermal excitation by means of current pulses through resistancesthat are diffused in the surface generates oscillations, for which theoscillation plane is at a right angle to the resonator structure,thereby resulting in a number of disadvantages. In particular, theachievable temperature difference between front and rear side of theheated web is very low, owing to the spatial nearness and the poorthermal insulation. Thus, a relatively high heating output is required.Furthermore, the sensitivity direction of this sensor is perpendicularto its surface. This is not desirable for a plurality of applications,in particular for the use as acceleration sensor for passengerprotection systems in motor vehicles. However, forproduction-technological reasons, vertically oscillating resonators arenot very suitable for use as locally sensitive sensors because adifferent thickness is required, for example, for the resonator and theseismic mass.

SUMMARY OF THE INVENTION

It is the object of the invention to specify a microsensor with aresonator structure, for which a lateral thermal excitation of theoscillations occurs.

This object generally is solved according to the present invention witha microsensor having a resonator structure that is excited by firstelectrical signals to oscillate and which emits second electricalsignals in dependence on a sensed measuring variable, wherein for thethermal excitation of oscillations, a heating element is arranged on theresonator structure, which heating element is supplied with at least oneof the first electrical signal, at one base connector point of at leastone oscillating segment or portion of the resonator structure, themicrosensor has at least two regions that are thermally separated by azone with reduced heat conductance and the heating element is arrangedon one of the two regions, and excites the resonator structure tolateral oscillations. Advantageous embodiments and modifying of themicrosensors are disclosed and discussed.

In the case of a microsensor with a resonator structure and thermalexcitation of the oscillations, at least two regions that are thermallyseparated by a zone with decreased heat conductance are provided at onebase connection point of at least one oscillating segment of theresonator structure, with the heating element being arranged on one ofthe region. As a result of this arrangement, lateral oscillations can beexcited in the resonator structure if the heating element is providedwith corresponding current pulses. It is advantageous if a recordingelement is arranged at least one of the other regions to detect theoscillation amplitude.

The heating element for one advantageous embodiment of the invention isused simultaneously for the detection of the oscillation amplitude.

A first embodiment of the invention provides that the zone with reducedheat conductance comprises a mechanical recess, which produces anincreased thermal insulation between the resulting two regions.

A second embodiment of the invention provides that the oscillatingportion of the resonator structure consists of a resonator web, fixed atone or several locations, which changes to a U-profile at its baseconnections point. In this case, the heating element is arranged on thefirst leg of the U-profile and the receiving element on the second legof the U-profile.

In order to sense the measuring variable, it is advantageous if theresonator web is fixed with the end facing away from the U-profile to asensor-specific structure, which leads to a detuning of the resonancefrequency. In the embodiment as acceleration sensor, this is a seismicmass.

Another embodiment of the invention provides that the oscillating partof the resonator structure consists of a tuning fork, having a recessarranged near the base point. This recess provides a thermal insulationbetween a web that connects the tins of the tuning fork and the basepoint of the turning fork. The heating element in that case is arrangedadvantageously on the web.

The resonator structure consists of a semiconductor material, preferablya mono-crystalline silicon. The heating element is designed as dopedresistance zone in the silicon material or as thin-film resistor on thesurface of the resonator structure.

The receiving element for detecting the oscillation amplitude isdesigned as piezoresistance. A capacitive detection of the oscillationamplitude is also possible.

BRIEF DESCRIPTION OF THE DRAWING

Short description of the Figures:

FIG. 1 shows an acceleration sensor according to the invention in a viewfrom above.

FIG. 2 shows a detail of the acceleration sensor in FIG. 1.

FIG. 3 shows a further detail of the acceleration sensor in FIG. 1.

FIG. 4 shows a tuning fork resonator according to the invention.

FIG. 5 shows the acceleration sensor in FIG. 1, in a perspective view.

FIG. 6 shows a sectional detail of FIG. 2, in a perspective view.

FIG. 7 shows a sectional detail of FIG. 3 in a perspective view.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention relates to the basic structure of micro-mechanicallyproduced, thermally activated resonators with a lateral oscillationdirection (relative to the chip plane). The idea behind the invention isto be explained with the aid of an acceleration sensor with resonantsignal reading and lateral sensitivity direction. However, this isintended to be an exemplary embodiment only and the invention is in noway limited to this embodiment. Further applications for the inventionare in the sensor technology field, e.g. for sensing pressure, force,and speed, as well as for the chemical sensor technology, the frequencynormal and the filtering technology.

The basic resonator structure is characterized in that it has amechanical recess in or near its base point, which creates an increasedthermal insulation between the resulting two mechanical partial regions.FIG. 1 shows a micro-mechanical acceleration sensor of this type, inparticular its resonator structure, in a view from above. The sensor 1essentially comprises a chip frame 2 and a seismic mass 3, which ispositioned such that it can oscillate inside the chip frame and whichacts upon the oscillating portion or segment of the resonator structure,the resonator structure, the resonator web 5, by way of a lever 4. Dueto the fact that the lever 4 in present exemplary embodimentsimultaneously represents the clip frame 2. A U-profile is arranged atthe base connection point of the resonator web 5, meaning where theresonator web is connected to the clip frame 2. Its two legs or regionsare connected here to the clip frame 2 and the resonator web 5 isconnected to the center of the crossbeam of the U-profile.

In the concrete case of the acceleration sensor, the two legs are tworegions that are thermally separated by the recess 8 of the U-profile.The recess 8 forms a zone with reduced heat conductance.

FIG. 2 provides a detailed view of this portion of the resonatorstructure. A resistor 10 is deposited on one of two region as a heatingelement and a receiving element a for detecting the oscillationamplitude is deposited on the other of the two regions. The receivingelement a in the present exemplary embodiment is a piezoresistance.However, other and for example capacitive, receiving elements can beused advantageously for this. The oscillation place for the resonatorweb 5 is in the plane for the resonator structure, i.e., lattered and isindicated in FIG. 2 with the dashed lines showing the deflectionposition 5′ for the resonator web 5. A periodic excitation of theheating element 10 by a pulsed current will lead to a thermaldeformation of this leg in the excitation cycle. The resonator web 5 isexcited and starts oscillating at the same time. FIG. 3 shows theU-profile 7 once more in an enlarged representation.

Another option is to respectively deposit one heating element on bothpartial structures and to activate both in the push-pull mode. Thereceiving elements must then be repositioned accordingly.

In the simplest case, the heating resistance 10 is realized by re-dopinga zone in the semiconductor material of the resonator structure. Theadvantage in this case is a very easy production, as well as the factthat no unnecessary temperature drifts are induced (e.g. as for thebimetals). However, it is also possible to deposit a thin-film heatingresistance at the respective location on the resonator structure.

In the simplest case, the oscillation amplitude can be detected via apiezoresistance as the receiving element 9. If acceleration acts uponthe seismic mass 3, the resonant frequency of the resonator web 5 willalso change as a result of the change in the mechanical fixation of theresonator web 5, owing to the effect of the lever 4. The output signalfrom the microsensor is the resonance frequency that is proportional tothe acceleration, which is determined by means of a suitable electroniccircuit that comprises the heating element and the receiving element.

The FIGS. 5, 6 and 7 again show a perspective view of the microsensor inthe respective detailed views according to FIGS. 1, 2 and 3. Theelectrical feed lines for the heating element and the receiving elementare also shown therein.

A tuning fork structure for measuring rotational speeds is shown in FIG.4 as a second exemplary embodiment. A mechanical connecting web 12 isrealized there between the two tins 13, 13′, which is separated from thebase connection point 14 of the tuning fork tins by a mechanical recess15. The heating resistance 16 is located on this connecting web 12. Thepush-pull mode of the tuning fork is excited with this geometry, whichmode is necessary for measuring the rotational speed by means of theCoriolis force. Capacitive measuring electrodes or piezoresistances,which are not shown in the Figure for reasons of clarity, can be used toread the amplitude. If necessary, the person skilled in the art canadjust these based on the sensor requirements and can place themaccordingly on the surface.

Another example involves a pressure sensor for very low pressures (<10mbar), in particular for the vacuum technology. A simple resonator webcan be used for this. The resonant rise or the quality of the resonantfrequency, which depends strongly on the environmental pressure, is usedas measuring variable. The sensor can be adapted to the desired pressurerange through a corresponding selection of the oscillating masses or thecross-sectional surfaces.

Another exemplary embodiment is a simple frequency normal formicro-electronic circuits, corresponding to an oscillator crystal. Theresonator structure can be operated without problems in the range ofseveral 100 kHz, up to the MHz range. Owing to its simple design, it ispossible to integrate the resonator directly into an IC (integratedcircuit) and thus avoid the necessity of oscillating crystals.

What is claimed is:
 1. A microsensor having: a resonator structure thatis excited by first electrical signals to oscillate and which emitssecond electrical signals in dependence on a sensed measurementvariable, and wherein: for a thermal excitation of oscillations, aheating element is arranged on the resonator structure, which heatingelement is supplied with at least one of the first electrical signals;at one base connection of at least one oscillating segment of theresonator structure, the oscillating segment has at least two regionsthat are thermally separated by a zone with reduced heat conductance;and the heating element is arranged on one of the two regions andexcites the resonator structure to lateral oscillations.
 2. Amicrosensor with a resonator structure according to claim 1, wherein areceiving element for detecting the oscillation amplitude is arranged onat least one of the other of the two regions.
 3. A microsensor with aresonator structure according to claim 1, wherein the zone with reducedheat conductance contains a mechanical recess, which creates anincreased thermal insulation between the resulting two regions.
 4. Amicrosensor with a resonator structure according to claim 1, wherein theresonator structure is composed of mono-crystalline silicon and theheating element is a doped resistance zone in the silicon material.
 5. Amicrosensor with a resonator structure according to claim 1, wherein theheating element is a thin-film resistor on the surface of the resonatorstructure.
 6. A microsensor with a resonator structure according toclaim 1, wherein the oscillating segment of the resonator structureconsists of a resonator web that is fixed at one or several locationsand changes into a U-profile at its base connection point, wherein theheating element is located on a first leg of the U-profile and thereceiving element is located on a second leg of the U-profile.
 7. Amicrosensor with a resonator structure according to claim 6, wherein theresonator web is fixed at the end facing away from the U-profile to asensor-specific structure, which leads to a detuning of the resonancefrequency of the said resonant structure.
 8. A microsensor with aresonator structure according to claim 7, wherein the sensor-specificstructure for detuning the resonance frequency is a seismic mass.
 9. Amicrosensor with a resonator structure according to claim 1, wherein theoscillating segment of the resonator structure consists of a tuningfork, provided with a recess near the base connection point of the tins,which recess thermally insulates a web that joins the tins of the tuningfork from the base connection point.
 10. A microsensor with a resonatorstructure according to claim 9, wherein the heating element is arrangedon the web.
 11. A microsensor with a resonator structure according toclaim 1, wherein a resonator structure consists of a mono-crystallinesilicon.
 12. A microsensor with a resonator structure according to claim2, wherein the receiving element is a piezoresistance to detect theoscillation amplitude.
 13. A microsensor with a resonator structureaccording to claim 1, wherein a capacitive sensor for detection of theoscillation amplitude is provided.